Characterization of Open-Cycle Coal-Fired MHD Generators. Quarterly Technical Summary Report No. 6, October 1--December 31, 1977. [Package Code] Page: 61 of 210
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This can also be expressed as an ionization efficiency of 1 x 10-3A/torr.
Extranuclear designs its ionizers for especially high efficiency, due to factors not
considered in these simple calculations. They state that their efficiencies are on
the order of 10 to 10-4 A/torr, with the smaller number being more likely in
normal operation. If we accept this figure, and further assume loss through the
quadrupole mass filter on the order of a factor of ten, we would expect a current
entering the particle multiplier of 10-12A. The particle multiplier has a gain of
the order of 106. We observe the current output by it across a load resistor of
5 x 105 ohms. Therefore, we might expect a peak height of 0.5 volts, and indeed
this is precisely the nitrogen peak height commonly observed on the oscilloscope.
A parameter of some importance which did not enter the preceding discussion
of beam intensity is the distance between the sampling nozzle orifice and the skimmer.
The exact theory of the dependence of beam transmission on this distance is not well
understood, but its effect on signal-to-noise ratio is easily explained. Figures 4.17
and 4.18 give the basis for sensitivity of detec tion of a modulated molecular beam.
The noise in the modulated spectrum of Figure 4.17 is primarily due to fluctuations
in the background gas pressure. The background gas is ionized along with the beam,
and fluctuations which happen to be near the chopping frequency are detected by the
lock-in amplifier. (Since the ionizer is turned off when detecting flame ions, this
source of noise is removed.) Figure 4.18 shows the background gas mass spectrun
which must be discriminated against.
From this discussion we see that an increase in beam signal means almost an
equal increase in signal-to-noise ratio. The improvement resulting in increased beam
signal was prompted by the receipt of a report on a very similar atmospheric
pressure molecular beam mass spectrometer apparatus set up at NASA-Lewis
Research Center (7). They had done studies of beam intensity as a function of sampling
orifice to skimmer orifice distance. We had found such curves before in the
literature, but only up to a few hundred torr, while the NASA-Lewis results extended
to one atmosphere source pressure. With this new information, we moved the skimmer
back to a position 1.25 inches from the sampling orifice. This resulted in an increase
in signal of a factor of five to ten, which since it did not contribute to background gas4-32
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Kolb, C. E.; Yousefian, V.; Wormhoudt, J.; Haimes, R.; Martinez-Sanchez, M. & Kerrebrock, J. L. Characterization of Open-Cycle Coal-Fired MHD Generators. Quarterly Technical Summary Report No. 6, October 1--December 31, 1977. [Package Code], report, January 30, 1978; Bedford, Massachusetts. (https://digital.library.unt.edu/ark:/67531/metadc1207188/m1/61/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.